Measure very high current spikes with a DIY low inductance coax shunt

[RoGeorge]

That's not 4ka. That's induced voltage. To accurately measure fast rising current, you need a current clamp or a high bandwidth coaxial current shunt.

I do a lot "few ka/us" measurements (my day time job is to design wide band gap power devices application and testers), and learned this the hard way. Sometimes even very small 1206 shunt can give you nasty inductive voltage.

The poor man's current shunt can be made out of a piece of coax cable, and short circuit inner and outer conductor on one end (current in), while on the other end, connect your scope between inner core and outer shielding. Current goes out from outer shielding.

What you get from scope is only resistive drop on outer shielding. Inductive voltage will be cancelled out by inner conductor.

The quoted message is from another thread, and it's about possible fake peak current measured because of the inductance of a normal resistive shunt and the ringing of the shunt while measuring a current spike.

The proposed DIY shunt seems like a very clever trick to distinguish between shunt inductive voltage ringing of the measuring current shunt and its resistive voltage drop.
I'm not sure I correctly understand it. Also googling about coax shunts doesn't helped me much with an explanation. Maybe an equivalent schematic will help.

Is this suppose to act like a 1:1 transformer, where the shield would be the primary, and the inner cable would be the secondary of the transformer?
Isn't the EM field inside the shield supposed to be zero, so it will be no induced voltage in the inner cable?

[alm]

EDIT: This information was wrong.

See this app note for some pretty pictures .

[RoGeorge]

This is how I understood the described DIY non inductive shunt from the blueskull's post. Did I get it right?

[alm]

That is not my interpretation. It would not make sense: barely any current would flow through the center conductor, which also means barely any magnetic field. See attached sketch for what I mean.

[RoGeorge]

Thanks for the schematic, alm.

I excluded that kind of shunt you drawn because, if I follow exactly the steps from the quoted message, then I will end up with a shunt where the measured current pass only through the out shield, without including the inner wire in the measured current path. Also, the original message is talking about cancelling the induced voltage, not just minimizing it. Cancelling (for a shunt like you drawn) will imply that the inner wire and the outer shield would have the same R and L, which I doubt it is true, so I excluded the schematic where the inner wire and the shield are in series.

If the shunt described by blueskull is like in your schematic, then it's pretty clear.

Maybe blueskull will see this thread and confirm which schematic he was described.

[alm]

PM him if you want his opinion. Did you look at the app note I linked to?

[mikerj]

This is how I understood the described DIY non inductive shunt from the blueskull's post. Did I get it right?

That's exactly how I interpret it.

[RoGeorge]

PM him if you want his opinion. Did you look at the app note I linked to?

I already PM him. Yes. First I just looked at it, but since the shunt from the app was different from the one I drawn, I didn't actually read it.

Your question made me open the app note again and actually read it. That PCB implementation of a low inductance shunt is very clever, thank you!

Also, maybe I should have not mention about equal R and L of the inner wire and shield as a condition for cancelling the inductance of the shunt, because it's the cancelling of the field that matters, not the R and L, so thanks again.

Now, back to the coax DIY shunt, judging by the field cancellation criteria, a coax shorted at one end seems like a pretty poor choice for field cancellation when compared to two conductive tubes one inside the other (and very near to each other, like in the industrial coaxial current shunts).

[alm]

True, it will probably perform considerably worse. Using a triaxial cable and ignoring the center conductor might work better (but less convenient). Rereading blueskull's quoted post, it does seem to match your schematic. But that schematic does not really make sense to me. Would you not also want to cancel the effect of the inductance on the sense wire? Would it not still mess up your circuit?

[2N3055]

blueskull's method doesn't try to cancel out magnetic field.
Inner conductor is basically a secondary on a 1:1 transformer (inner conductor and shield share magnetic field), and voltage pulse on the primary (shield) is differentially added (subtracted) with secondary winding (inner conductor) to form a signal going to scope...